This type of read-through is less important for ad ROI calculation, but I want to talk about it for a moment, nonetheless.
This is the RT between one book and the next. For example, if you have 45 sales in a given month on book 3 in your series, and 38 sales of book 4, you divide the later book’s sales by the earlier book’s sales to get your read-through percentage.
38 ÷ 45 = 0.84
84%
Therefore, the Book-over-book read-through (BOB RT) between books 3 and 4 is 84%.
That would be a decent RT, but perhaps a bit on the low side this late in the series. Most of the time, folks see RT in the 90% – 95% range later in their series.
Here’s an example of a healthy BOB RT:
Book 1 –> Book 2: 50% to 75%
Book 2 –> Book 3: 80% to 95%
Book 3 –> Book 4: 85% to 98%
Book 4 –> Book 5: 90% to 100%
If you see a particular book suddenly fall off a cliff in regards to BOB RT, then you know that you’ve made a mistake. You probably killed off a beloved character and lost part of your readership.
Chances are, your book’s reviews will tell you what you did wrong.